A fuel cell comprising a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a pair of electrodes, i.e., an anode and a cathode, formed on both surfaces of a hydrogen-ion conductive polymer electrolyte membrane. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a metal catalyst such as platinum group metal catalyst and a diffusion layer having both gas permeability and electronic conductivity formed on the outer surface of the catalyst layer.
In order to prevent the fuel gas and oxidant gas supplied to the electrodes from leaking out or prevent these two kinds of gases from mixing together, gaskets are arranged on the periphery of the electrodes in such a manner as to sandwich the polymer electrolyte membrane. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. The separator plates have, at a portion to come in contact with the MEA, a gas flow channel formed for supplying a reaction gas to the electrode surface and removing a generated gas and an excess gas. Although the gas flow channel may be provided separately from the separator plates, grooves are commonly formed on the surfaces of the separator plates to serve as the gas flow channel. Also, since the conductive separator plates are required to have high electronic conductivity, gas tightness and high corrosion resistance, it has been a common practice to process a dense carbon plate or the like to form grooves thereon by cutting, etc., for producing a separator plate.
In the conventional conductive separator plates, the gas flow channel formed thereon is usually of the straight type in which a plurality of linear gas flow channels extend in parallel from the gas inlet toward the gas outlet. In the polymer electrolyte fuel cell, however, water is generated on the air electrode side during its operation, and thus efficient removal of the generated water is essential for the fuel cell to exert its full performance. Therefore, in an attempt to remove the generated water forcibly, the cross sectional area of the gas flow channel of the conductive separator plate is reduced, and the gas flow channel is caused to meander to constitute a serpentine structure in order to increase the length of one gas flow channel, thereby to increase the gas flow rate substantially.
In the actual use of the fuel cell, a large number of unit cells as describe above are usually stacked to constitute a laminated structure. Since the fuel cell generates heat as well as electric power during its operation, a cooling plate is inserted every one to two unit cells in the cell stack to keep the cell temperature constant, while thermal energy produced is utilized, for example, in the form of hot water. The cooling plate commonly has a structure of circulating a heating medium such as cooling water through the inside of a thin metallic plate, but the cooling plate may also have another structure of forming a cooling water flow channel on the backside of the separator plate constituting the unit cell. In this case, an O-ring or a gasket is needed for sealing the heating medium such as cooling water. In such sealing, the O-ring placed between the cooling plates needs to be compressed completely in order to secure sufficient conductivity between the cooling plates.
In such a cell stack, apertures called manifolds are formed in the separator plates in order to inject the fuel gas, oxidant gas and cooling water to each unit cell. A common type is called internal manifold, which has apertures for supplying and discharging cooling water inside the cell stack.
In either case of using the internal manifold or external manifold, it is necessary that a plurality of unit cells including cooling sections are stacked in one direction, that a pair of end plates are arranged at both ends of the stacked cells, and that the two end plates are fixed by clamping rods. As to the clamping, the unit cells are desirably clamped as uniformly within the unit cell as possible. In view of the mechanical strength, the end plates and the clamping rods are usually composed of a metallic material such as stainless steel. These end plates and the clamping rods are electrically insulated from the cell stack by insulating plates so as to constitute a structure where the current is prevented from leaking out through the end plates. As for the clamping rod, it has been suggested that the rod is passed through a through-hole of the separator plate or the whole cell stack including the end plates is clamped by metal belts.
In the aforementioned polymer electrolyte fuel cell, the electrolyte membrane, when humidified with water, functions as an electrolyte, and therefore the fuel gas and the oxidant gas to be supplied need to be humidified before being supplied. In the polymer electrolyte membrane, the ionic conductivity increases with increasing water content in a temperature range of up to at least 100° C., so that the internal resistance of the cell is reduced to improve the output. Thus, in order to increase the water content of the electrolyte membrane, the supply gases need to be highly humidified before being supplied.
However, supplying the highly humidified gases above the cell operating temperature causes condensation of water within the cell, so that the water drops disturb smooth supply of the gas; on the air electrode side to which the oxidant gas is supplied, water generation as a result of electric power generation lowers the removal efficiency of the generated water, thereby deteriorating the cell performance. This is why the gases are usually humidified so as to have a dew point equal to or lower than the cell operating temperature and are supplied.
The supply gases are generally humidified by a bubbler humidification system where a supply gas is caused to bubble up in deionized water maintained at a predetermined temperature for humidification or a membrane humidification system where deionized water maintained at a predetermined temperature is flown to one side of an electrolyte membrane while a supply gas is flown to the other side for humidification. When a gas prepared by steam reforming a fossil fuel such as methanol or methane is used as the fuel gas, such humidification may be unnecessary in some cases since the reformed gas contains steam.
The humidified fuel and oxidant gases are supplied to the polymer electrolyte fuel cell and used for electric power generation. At this time, within a unit cell of the cell stack, current density distribution occurs. To be more specific, while the fuel gas humidified in a predetermined manner at the gas supply inlet is supplied, hydrogen in the fuel gas is consumed for electric power generation, so that the more upstream of the gas, the higher the hydrogen partial pressure becomes and the lower the steam partial pressure becomes. Accordingly, the more downstream of the gas, the lower the hydrogen partial pressure becomes and the higher the steam partial pressure becomes.
Also, while the oxidant gas humidified in a predetermined manner at the gas supply inlet is supplied, oxygen in the oxidant gas is consumed for electric power generation and water is generated as a result of electric power generation. Consequently, the more upstream of the gas, the higher the oxygen partial pressure becomes and the lower the steam partial pressure becomes, whereas the more downstream of the gas, the lower the oxygen partial pressure becomes and the higher the steam partial pressure becomes. Moreover, with respect to the temperature of cooling water for cooling the cells, the closer to the inlet, the lower it becomes, whereas the closer to the outlet, the higher it becomes, so that temperature distribution occurs within the unit cell. This is why current density distribution occurs within the unit cell.
As described above, when the unevenness of the hydrogen and steam partial pressures in the fuel gas, the unevenness of the oxygen and steam partial pressures in the oxidant gas, the temperature distribution, etc., become excessive within the unit cell, the cell performance is greatly deteriorated due to excessive dryness or overdry, or excessive wetness or overflooding.
Further, a phenomenon of coexistence of the overdry and the overflooding within the unit cell occurs also due to the unevenness of the hydrogen and steam partial pressures in the fuel gas, the unevenness of the oxygen and steam partial pressures in the oxidant gas, the temperature distribution within the unit cell, etc., which are caused by the above-mentioned reasons.
In a stack of a large number of cells, occurrence of the above problem in a part of the cell stack may interfere with the operation of the whole cell stack. Specifically, when a part of the cell stack becomes overflooded, the loss of gas supply pressure increases in the overflooded cell. Since the manifolds for gas supply are shared in the cell stack, the gas does not flow smoothly into the overflooded cell, consequently increasing the overflooding.
On the other hand, when a part of the cell stack becomes overdried, the loss of gas supply pressure decreases in the overdried cell. Thus, the gas flows smoothly into the overdried cell, consequently increasing the overdry.
The above-described problem often results from the fact that the steam partial pressure in the gas is higher on the gas outlet side than on the gas inlet side on both the fuel electrode side to which the fuel gas is supplied and the air electrode side to which the oxidant gas is supplied. Therefore, as disclosed in Japanese Laid-Open Patent Publication No. Hei 9-511356, the flowing direction of the oxidant gas was made the same as that of the cooling water to make the downstream temperature of the oxidant gas higher than the upstream temperature due to temperature distribution of the cooling water, in an attempt to suppress the overflooding downstream of the air electrode and reduce the current density distribution within the unit cell.
The polymer electrolyte fuel cell, when used as the electric vehicle power source, is required to be compact, lightweight and inexpensive. Also, when used as the domestic cogeneration system, it is required to be compact, highly efficient and inexpensive. In either case, the fuel cell is intended to be used with a reformer, supply gas humidifier, exhaust heat recovery/converter/inverter, etc., as a system; in order to make the whole system more compact, the polymer electrolyte fuel cell is required to be more compact, and limitations are imposed on the shape of the installation space of the cell. The requirement for a thinner cell becomes stronger especially when the cell is installed in a lower part of the body of an electric vehicle as the power source.
Further, when the conductive separator plate is composed of a carbon material, the process of forming a gas flow channel by cutting is costly; thus, an attempt has been made in which a carbon powder, carbon fiber or the like is mixed with a resin and is molded by a process such as hot pressing without baking it at a high temperature. Such molded carbon, however, has a disadvantage that it has a lower mechanical strength and is more fragile than the baked carbon separator.
Meanwhile, when the polymer electrolyte fuel cell is used for a relatively small-sized cogeneration system such as domestic cogeneration system, the power of auxiliaries, for example, for supplying air has a major influence on the efficiency of the whole system. Thus, in order to reduce the power of a blower for supplying air to the air electrode side or other auxiliaries, there is a need to reduce the pressure loss in supplying air to the air electrode. The reduction of the pressure loss on the air electrode side requires enlargement of the cross sectional area of the gas flow channel in the air-electrode-side separator plate; from this viewpoint, the serpentine type flow channel is inadequate for the small-sized cogeneration system. However, the inventors of the present invention have found that in the case where the portion of the conductive separator plate in contact with the electrode has a shape like a square or circle, the cell is unable to exert its performance sufficiently if the gas flow channel on the air electrode side is of the straight type. This is because it is not possible to increase the gas flow rate sufficiently. Then, in order to increase the gas flow rate while using the straight type flow channel, the inventors have tried reducing the depth of the gas flow channel to find that when the depth is less than 0.4 mm, the gasket and diffusion layer of the electrode partially fall into the gas flow channel to undesirably hamper or inhomogeninize the flow of the gas.
Also, in the case where the portion of the separator plate in contact with the electrode has a rectangular shape, if the length of the longer side is equal to or more than six times that of the shorter side and the gas flow channel on the air electrode side is of the serpentine type, the loss of gas supply pressure becomes too large. When the pressure loss at the gas inlet becomes large, the relative humidity at the gas inlet becomes too high in comparison with that of the gas outlet, so that the cell is unable to exert its performance sufficiently.
Further, another problem arises that depending on the cross sectional area of the gas flow channel, the loss of gas supply pressure becomes large and necessary auxiliary power accordingly becomes too large. Then, when the cross sectional area of the gas flow channel is enlarged to reduce the loss of gas supply pressure, if the width of the gas flow groove is enlarged, the actual contact area of the separator plate and the electrode is reduced to increase the contact resistance. Also, the interval between the portions supporting the electrode becomes wider, thereby to increase the contact resistance between the electrode and the electrolyte membrane. If the depth of the groove of the gas flow channel is enlarged, the thickness of the separator plate is inevitably enlarged, making it impossible to make the whole cell compact. Further, other problems arise that the diffusion of the supply gas throughout the electrode surface is hindered and the gas utilization efficiency is aggravated, thereby to increase the reaction resistance of the electrodes.
Furthermore, in order to make the whole fuel cell stack more compact, it is indispensable to increase the ratio of the effective area of the electrode to the area of the separator plate. For such purpose, when a rectangular conductive separator plate is used, the portion of the separator plate in contact with the electrode inevitably has a rectangular shape in correspondence with the shape of the separator plate.
The inventors of the present invention have made various examinations of the rectangular conductive separator plate and found the followings.
When the linear part of the gas flow channel is arranged in parallel with the shorter sides of the rectangular separator plate, condensed water such as generated water cannot be removed efficiently, so that the cell is unable to exert its performance sufficiently. In the construction of the serpentine flow channel, in particular, when the linear part of the flow channel is arranged in parallel with the shorter sides of the rectangular separator plate, the flow channel makes more turns than when the linear part of the serpentine flow channel is arranged in parallel with the longer sides of the rectangular separator plate, so that the pressure loss is increased and the removal efficiency of water or generated water in the gas is deteriorated even when the flow channel is formed to have the same cross sectional area, which results in deterioration of the cell performance.